Interferometric Radio Science

1. Interferometric Radio Science Tiziana Venturi INAF, Istituto di Radioastronomia 4th ERIS, Rimini, 5 September 2011

2. Radio Astronomy at the cutting-edge of astrophysical research • Roughly 70% of what we know today about the Universe and its dynamics is due to radio astronomy observations, rather than optical observations (from a presentation of Marcus Leech)

3. Outline Very general introduction to Radio Astronomy & introduction to the 4th ERIS • Radio waves • Angular resolution and need for interferometry • Phase of the visibility function • The u-v plane • Mechanisms for radio emission in astrophysics • The syncrotron radio spectrum • New and upcoming facilities in the Northern and Southern Hemispere • The 4th ERIS

4. Radio Astronomy: wavelengths from a few mm to tens of meters Visible light: wavelengths in the region of 500nm, (5.0x10-7 m) From a physics standpoint, there's no difference between visible light, and microwave/radio-wave “light”.

5. Optical and Radio can be done from the ground NRAO/AUI/NSF 5

6. Why radio interferometry θ~λ/ D Ability to resolve fine detail highly dependent on wavelength A 10cm optical telescope can resolve details that would require a radio telescope over 42km in diameter at 21cm wavelength! Earth rotation synthesis + Sensitivity, however, is proportional to collecting area of the reflector, regardless of wavelength

7. Angular resolutions at 20 cm (1.4 GHz) Effelsberg GMRT Connected elements EVLA D-array D=100m θ~ 9.4’ D=1km θ~ 44” D=28km θ~ 1” EVN D=217 km θ~ 150 mas D~10000 km θ~ 5 mas θ≈fractionof mas

8. HST θ~ 50 mas (angular resolution of eMERLIN at 5 GHz) Chandra θ~1” (angular resolution of the EVLA Array A and of the GMRT at 1.4 GHz)

9. Overlay of the radio-optical & X-ray emission in a cluster of galaxies Red Chandra Green GMRT at 610 MHz Optical DSS-2

10. Overlay of the radio-optical & X-ray emission in Centaurus A

11. Image of Cygnus A at l = 6cm. • The units are in Jy/beam. • 1Jy = 10-26 watt/(m2 Hz) • Here, 1 beam = 0.16 arcsec2 Imaging in astronomy implies ‘making a picture’ of celestial emission. We design instruments to make a map of the brightness of the sky, at some frequency, as a function of RA and Dec. In astronomy, brightness (or specific intensity) is denoted In,t(s). Brightness is defined as the power received per unit frequency dn at a particular frequency n, per unit solid angle dW from direction s, per unit collecting area dA. The units of brightness are in terms of (spectral fluxdensity)/(solid angle): e.g: watt/(m2 Hz Ster) From R. Perley 2010

12. Main Issues with interferometric observations Eachpairofantennas in aninterferometeris a baseline Phase corruption Calibration u-v coverage Deconvolution & Imaging

13. Amplitude carries information on the source intensity Phase carries information on the source absolute position

14. Phaseerrors depend on the electronics, and on the different propagation paths of the radio signal through the atmosphere, which introduce an unknown quantity in the phase, which differs from telescope to telescope Amplitude uncertainties and errors depend on the individual antennas and receivers

15. Changed positions & distorted sources Ionosphere at low frequencies (and man-generated RFI !!!)

16. Absorption bands at high frequency in the troposhpere

17. The u-v plane • A radio interferometer array can be considered as a partially filled aperture • - each pair of antennas gives a u-v point at a given time; • the point source function (PSF, or beam) has a complicated structure, which depends on the array, source declination and u-v coverage; • the u-v plane shows what part of the aperture is filled by a telescope, and this changes with time as the object rises and sets; • a long exposure will have a better PSF/beam because there is better u-v plane coverage (closer to a filled aperture)

18. The u-v plane is a plane tangential to the source in the celestial sphere. Each point on that plane is the projection of a baseline at a given time. Each pair of radio telescopes produces a track in the u-v plane. The number of tracks is equivalent to N(N-1)/2, where N is the number of radio telescopes in the interferometer.

20. ATCA – 1.4 GHz Res. ~ 10”x5” rms ~0.15 mJy/b Southern Cluster of galaxies A3562 GMRT – 610 MHz Res. ~ 8”x6” rms ~ 0.08 mJy/b

21. Southern Cluster of galaxies A3562 GMRT – 610 MHz Res. ~ 8”x6” rms ~ 0.08 mJy/b GMRT – 610 MHz Res. ~ 30”x20” rms ~ 0.14 mJy/b

22. Dirty Beams: A snapshot (few min) Full 10 hrs VLA+VLBA+GBT

23. Sequence from Amy Mioduszewski (NRAO’s 2010 Synthesis Imaging Workshop)

36. What do we look at when we observe at radio frequencies? Main mechanisms for radio emission - Thermal properties - Ionized medium (T and ρ) - Composition and properties (T and ρ) of the ISM/IGM - Relativistic electrons and magnetic fields • - Blackbody radiation • Cosmic Microwave Background • - Thermal Bremsstrahlung • - Spectral lines from molecular and atomic gas clouds • - Synchrotron radiation

37. Black body & Bremsstrahlung radiation • Emission from warm bodies • “Blackbody” radiation • Bodies with temperatures of ~ 3-30 K emit in the mm & submm bands • Emission from accelerating charged particles • “Bremsstrahlung” or free-free emission from ionized plasmas

38. Neutral hydrogen (HI) line emission Emits photon with a wavelength of 21 cm (frequency of 1.42 GHz) Transition probability=3x10-15 s-1 = once in 11 Myr

39. Line emission Molecular vibrational and rotational modes • Commonly observed molecules in space: • Carbon Monoxide (CO) • Water (H2O), OH, HCN, HCO+, CS • Ammonia (NH3), Formaldehyde (H2CO) • Less common molecules: • Sugar, Alcohol, Antifreeze (Ethylene Glycol), … malondialdyde

40. Synchrotron radiation Polarized emission provides information on the magnetic field

41. Spectrum of the synchrotron radiation Turnover Optically thin S αν-α Opticallythick/Self-absorbed S αν2.5 Aged part of the spectrum due to radiative losses S αν-(α+k) Different parts of the synchrotron spectrum provide different information on the radio source and on the population of the radiating relativistic electrons

42. Example: an extragalactic radio source - 3C317 Steep spectrum dominated by the diffuse emission Concave component dominated by the VLBI active nucleus

43. Synchrotron radio sources and their spectra Radio galaxieson the kpc scale WNB1127.5+4927 3C296 3C452 From M . Murgia 2008

44. Synchrotron radio sources and their spectra Radio galaxieson the parsec scale

45. Synchrotron radio sources and their spectra Diffuse cluster sources

46. Present and future radio facilities Wide fields and the “weak” Universe New and upgraded observational facilities over the whole radio window are ready operational ALMA 10 bands from 35 to 850 GHz

47. Present and future radio facilities Wide fields and the “weak” Universe New and upgraded observational facilities over the whole radio window are ready operational EVLA Complete frequency coverage from 1 to 50 GHz ALMA 10 bands from 35 to 850 GHz

48. Present and future radio facilities Wide fields and the “weak” Universe New and upgraded observational facilities over the whole radio window are ready operational EVLA Complete frequency coverage from 1 to 50 GHz ALMA 10 bands from 35 to 850 GHz ATCA from 2 to 86 GHz

49. Present and future radio facilities Wide fields and the “weak” Universe New and upgraded observational facilities over the whole radio window are ready operational eVLBI and eMERLINfrom 1.6 to 22 GHz EVLA Complete frequency coverage from 1 to 50 GHz ALMA 10 bands from 35 to 850 GHz ATCA from 2 to 86 GHz

50. Present and future radio facilities Wide fields and the “weak” Universe New and upgraded observational facilities over the whole radio window are ready operational eVLBI and eMERLINfrom 1.6 to 22 GHz EVLA Complete frequency coverage from 1 to 50 GHz ALMA 10 bands from 35 to 850 GHz GMRT 1.4 GHz – 240 MHZ ATCA from 2 to 86 GHz